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How does the 'efficiency' of a magnus effect flier compare to helicopters and fixed wing aircraft?
I recently saw a demonstration of a flier based on the magnus effect. There is a good example on youtube here. How does the flying 'efficiency' compare to conventional designs?
When I say efficiency I'm not sure what the best measure is. Possibly lift to drag ratio?
I'm interested in:

lifting capacity: how much weight could a magnus effect flier lift vs a helicopter or fixed wing design

energy efficiency

Obviously these things depend on many factors like size of the airfoil & airspeed so a rough guide would be sufficient.
There is a related question - what-are-the-advantages-of-a-spinning-wing-magnus-effect. This discusses practicalities but does not really cover the relative lift produced.
I also have the same question about cyclorotors, which I have posted separately.

One thing is for sure: after more than a century of fligh all together, and the magnus effect, cycloidial rotors and whatnot being of a ripe age also, they are not very efficient when considering practical use. Otherwise they would be used extensively.

Solar and wind power were of a ripe old age without taking off not so long ago.  I'm looking for a more physics based comparison.

"Were it not for the wind, flying would be so easy"
The root issue with any "high lift" design is cruising airspeed.  The main advantage of commercial air travel is time savings.
Drag increases with V$^2$. As airspeed increases, drag from forward motion becomes much greater than drag from lift.
Airships (blimps or zeppelins) are most efficient at airspeeds up to around 80 knots.  They made a serious bid for success in the 1930s before a number of weather related crashes swung sentiment in favor of mutiengined aircraft capable of speeds 3 to 4 times greater.  These aircraft had a better safety record and were able to maintain schedules in more adverse conditions, critically, headwinds.
It is the cruise speed parameter, as efficient as possible, that one must design around. Modern aircraft simply fold their slats and flaps up to reduce drag at higher cruising speeds.  What would a "Magnus flyer" do?
Secondly, the gyroscopic effects of a rapidly rotating cylinder wing in the roll or yaw plane (try this with a spinning bicycle wheel) would likely end the commercial scale-up effort before long.
Although a Magnus cylinder would increase lift coefficient, increases in weight and drag at higher speeds, plus stability issues would have to be overcome.  In a world of biplanes, slats, flaps, slotted wings, blown wings, and helium, it is a non-starter.
Aside from gas, high aspect wings are most efficient for lifting.  Golf balls don't have wings, so they use ... the Magnus effect.

Regarding the gryoscopic stability issue. I would have thought you counteract that easily with something counter rotating.


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Does the nacelle strut experience reaction forces due to lift, drag, thrust, weight of engine during cruise flight?
Cruise flight indicates that total lift and weight of aircraft cancel and thrust forces generated cancels with total drag forces. However, I am looking closely at the nacelle instead of the whole aircraft. Would the strut that connects the nacelle to the wing still experience reaction forces during cruise flight? Since engine thrust generated is greater than engine drag and lift on the engine is larger than weight of engine it sounds like even during cruise there needs to be a reaction at the strut to fix the engine to the wing, unless I am misunderstanding something.

Related: How are engines mounted onto wings?. Indeed forces need to be transferred from the engine to the wing, the pylon strut needs to bear these forces, for each force there is an equal opposite reaction. The nacelle is neutral, it just hangs from the pylon.

Welcome to the site! Your choice of username is somewhat limiting.

It's OK, @GdD, we'll let him ask about other things, too. ;)

Yes.  The structure that joins the engine mount to the wing, nacelle, pylon, or whatever, has to transfer all of the forces being produced by the engine and propeller or fan, and their mass, to the wing box, and these forces are certainly present in cruise flight.
So you have the thrust of the propeller/fan applying a tension load forward pulling the airplane along, the torque reaction from the prop applying a rotation load trying to twist the nacelle, the weight of the engine and prop trying to bend the nacelle downward from gravity and G loads (where are there all the time unless the plane pitches over into zero G flight), and gyro precession loads from the propeller/fan trying to bend the nacelle to the side or up and down when the airplane yaws and pitches, and they would all be present to one degree or another during cruise flight.
You could eliminate the thrust and torque reaction loads by reducing power to remove those forces, but you can't do much to remove propeller/fan precession loads or gravity related loads while in flight.

"The nacelle structure that joins the engine mount to the wing has to transfer all of the forces", rather the pylon strut.

Actually wing mounted jet engines aren't "nacelles" anyway.  You have an engine, the cowl, and the pylon. A nacelle is normally a pod that fairs the engine into the wing and provides a structural base to attach the engine mount.  A pylon is not a strut or a nacelle.  Anyway, I tried to use a catch-all phrase, but made some changes to be more catch all.

The inlet, the duct for the fan flow, the reverser, the exhaust and the associated cowls make the nacelle. Here is the nacelle manual for CFM56-5 engines on the A320.

Ok fair enough.  We didn't call them nacelles on the CRJ program.

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Why is thrust (in a high bypass turbofan for e.g.) non-linear with respect to N1?
I've been studying jet engines for a short while now and I think I have a pretty decent understanding of how thrust is generated, but one thing I'm struggling a bit with is how thrust varies across the N1 range. More specifically, I have seen a few sources that state that it's more of an exponential relationship (i.e. going from 80 to 90% N1 results in a much larger increase in thrust than 50 to 60%), whereas if I had to guess I would have expected it to have been relatively linear (or at least something close to that).
What are the root causes of this non-linear behaviour?
If context is required, let's assume we are talking about modern day high-bypass turbofans on commercial aircraft (subsonic flight only).
*Edited to add a link to a source which figuratively shows what I mean by the relationship being exponential, this suggests Thrust to be roughly equal to N1^3.5 in this example:

Personally I would be more surprised if it were linear.  Torque and horsepower curves are always, well... curved.  And you can certainly feel a difference in the throttle response.  I don't have an engineering answer for you, but I would presume it has to do with efficiency at different RPMs.

Well to give you an example of what I was thinking, let's just look at bypass flow for now. I would have thought that the mass flow rate of the bypass air increases linearly with fan speed (fan law 1) and therefore as you increase N1 from idle to TOGA, the only thing that is changing is the MFR. As bypass accounts for a large portion of total thrust, I can't imagine that the core thrust is so non-linear that it results in total thrust being as it is. I guess my question is why is it extremely non-linear, for e.g. one source suggests that thrust is very roughly equal to N1^3.5

Well its definitely not exponential, but not linear either.  E.g. check out the graph in this answer: https://aviation.stackexchange.com/questions/37927/how-much-thrust-is-lost-due-to-the-bleed-air-system

I'm not intending to get caught up in semantics here so apologies in advance if it seems that way, but I've just recreated that graph in Excel and it does indeed give me an exponential trendline, specifically y = 681e^(0.037x). Thank you for the link though, that has helped as it is a real world example rather than the one I have now linked.

At its most fundamental the blade of the fan is an airfoil. Lift generated is proportional to the square of the relative wind speed. That’s non-linear.

I didn't think about it like that but that makes a lot of sense (and even seems obvious now!), thank you @Jim. It seems this question might have been much more complex than I thought, and I assume that there may well be other factors at play, but this is exactly the kind of answer I was hoping for to help point me in the right direction of where to go looking for more detail. I'm new to the site so still getting used to how it works but I personally would have accepted this as an answer if you wish to add it.

@Jim You should post this as an answer :-)

Gas turbine engines, e.g. a turbofan engine, are (is) designed for a specific operating point, for that point the engine operates most efficiently, any deviation from that point causes the air flow in the core of the engine to be of a different volume than the design point flow.
A compressor (or a fan) is designed for a certain compression ratio at a certain rotational speed. When the rotational speed changes (why? reducing the fuel flow causes less hot pressurized gas to be available to expand in the turbine, the less energy is available to compress air), air is compressed less and thus larger in volume. Knowing the passage of the compressor has a fixed cross section per compression stage, less compressed air causes air to flow faster through the stages. As a result the compressor velocity triangles change and blades have a different inflow and as such a different efficiency (lift generation, it can even trigger stall of the blades, unless countermeasures like bleed off-take are taken).
Compressors will work at other conditions than the design condition, the relation is usually displayed in a graph that we call the compressor characteristic (see e.g. What are the "beta lines" of a gas turbine engine component?), this characteristic relates (corrected) flow and pressure ratio to efficiency for given spool speed (using a fictive parameter called the beta lines to ensure a single solution for a given set of parameters is given). The further one moves from the optimum design point, the lower the efficiency (this is not a linear phenomena). Note that gas turbines are aerodynamically balanced and each compressor (or fan) is connected to a turbine (could be through a gearbox) which depends on a similar characteristic. The equilibrium that is reached determines how the engine operating point moves through the compressor characteristic.
When we sweep a fuel flow for a turbofan from high to low at a fixed flight condition (static on the ground), we can plot the results of such an experiment to show the effects of blades receiving the inflow at far less favorable conditions. E.g. the fan operating line:

We can take this further to create an image similar as posted in the question (note that the axes are swapped) for the fan spool speed N1 and the core spool speed N2:

Why the relation is not linear is that the performance depends on various aerodynamic effects (which are not linear) of various components.

A turbojet basically does something quite simple:

it gets some fresh air from the outside;
it compresses this air some 20 times so that its pressure and temperature raise;
when this hot and thick air comes into contact with kerosene, this latter just catches fire;
the temperature suddenly increases, as well as the volume of the mixture air/kerosene;
part of this suddenly expanding mixture impinges on the blades of a turbine making it spinning; the rest is ejected outside and pushes (thrusts) everything on the opposite direction.

Et voilà, fresh air has just been converted in thrust.

So, how do we extract some numbers from this indeed very simple process? Well, since we are dealing with air, we need for sure the tools of aerodynamics. Is it enough? No: this air reacts with kerosene creating new substances, so we need the tools of chemistry too. End of the story? Nope: in the process, air, kerosene and their products change pressure, temperature and density, so also the tools of thermodynamics are needed. And finally, the combustion products make the turbine rotates which, in turn, brings the compressor in rotation, so here the tools of mechanics are needed as well... Now, what started out as a very simple process, turned out, to be understood, to actually need all the knowledge from almost every known scientific branch!

What are the root causes of this non-linear behaviour?

Let's make a simple example where we analyse what happens in the combustion chamber when the throttle is set a bit back in order to decrease thrust:

the quantity of fuel reaching the combustion chamber reduces;
the combustion fuel/air reduces as well since there's less fuel to burn;
lower combustion means lower temperature and expansion of the combustion products;

the relation among the combustion energy and the temperature is quite simple $Q=mc_p∆T$ where $Q$ is the heat supplied by the combustion, $m$ is the mass of air+fuel, $c_p$ is the isobaric specific heat of air+fuel and $∆T$ the temperature change; this looks like a linear relation, double heat $Q$ double $∆T$; unfortunately $c_p$ depends itself in a non linear way on the pressure and temperature (see for example the plots here) and therefore the relation between combustion heat and temperature's raise is also not linear;
ok, temperature changes with the heat $Q$, what about the volume $V$? Even if we consider an ideal gas (that the combustion products are not), the relation is $V/T=\text{constant}$; so if the temperature changes of $∆T=T_2-T_1$, the volume changes of $∆V=V_2(1-T_1/T_2)$ which is also a not linear relation;

finally, from Newton's second+third law (aka momentum conservation) and from thermodynamics we get that the relation between the "push" transferred from the expanding combustion gases to the turbine is proportional to the square of the rotational speed $\omega$ i.e. $c_p∆T=K\omega²$ which is also not linear.

So, even only considering what happen in the combustion chamber we see that the relation among the several phisical variables involved in the process is in general not linear.

Any gas turbine engine is inefficient at low rpm. Why in technical terms I don’t know, maybe it is that the compression ratio required for efficient combustion is not achieved by a piston squeezing but by a continuous flow needing several stages to achieve. But it is certainly the nature of the beast. Throttling back below about 85% N2 (and the fan rpm is related to the core rpm) in the engines I operated from the PT6 to the PW4060 and the CF6-80 resulted in fuel inefficiency. That is why if you are short of fuel in a four engine plane you can shut one down. The three remaining have to work harder and the faster you spin them the more fuel efficient they are. Lockheed Orion marine patrol aircraft used to do this as SOP when patrolling down low. Similarly a modern twin airliner that has depressurised and has to fly at 10,000 ft uses more fuel on two engines than it does on one. If you used anywhere near normal cruise thrust down low on two engines you would overspeed the aircraft, and when you throttle back enough to keep the speed below max the engines come below their efficient rpm range. Not that you would do it on purpose, but if you were down to one engine in a twin down low, that engine has to work harder and that is what they like. Even considering the bit of extra drag from asymmetric flight and APU burn you will still use less fuel on one than two.

The question asks "Why".

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What causes ram air turbines with more blades to spin slower than turbines with two blades?
Lets say aircraft has two ram air turbines that work as generators to produce electricity on each wing. One has two blades and one has 8 blades, both with the same diameter.
What is the aerodynamic reason why the turbine with 8 blades will rotate slower (lower RPM) then the one with 2 blades?
Often people say each blade adds drag, so drag reduces RPM but where is aerodynamic drag on blades if the resultant force is always tilted forward? A component in the forward direction is thrust, not drag. So what slows down the rotation?
I think there can be only 2 reasons to slow down rotation:

if resultant force has lower magnitude or
the vector is pointed more backward, then the question remains why this change in resultant vector happened at more blades configuration?

The tip speed ratio is the ratio between tip speed tangential velocity and wind speed that blows at the propeller. So for same wind speed one blade has the highest RPM...

I don't think your assumption is valid.  Adding more blades ought to make the generator spin faster.

@quietflyer  propeler with more blades will spin slower, but has higher torque. And higher torque is direct indicator of higher drag, but I need aerodynamic explanation why this happened...

I'd suggest editing the question to include some external source supporting this idea that in generator mode, adding blades makes the prop spin slower.

Re lift and drag-- in relation to props, this can be confusing-- are lift and drag defined in relation to the direction of the external airflow, or in relation to the direction of the trajectory of a particular element on a particular blade?  Is the torque that turns the prop mainly due to drag, or mainly due to lift?  Anyway I'm still having trouble w/ this idea that adding more blades makes the generator spin slower.  Will be interesting to see what answers are posted.

DV & VTC - this question posits an unproven and intuitively questionable assumption & then asks why this unexpected result occurs. But, would it spin slower?  I don't see any productive outcome here. If you change the characteristics of the blades & the generator enough, then all sorts of outcomes are possible, but all "ceteris paribus" (all-else-being-equal) is out the window.

More blades = more obstruction of the wind = less airflow through the prop diameter = slower spinning but more torque. eventually with too many blades the power available(torque * hub speed) actually starts decreasing. It's elementary and well known and documented for wind turbines. Take the extreme case: infinite props=solid disk= stationary.

If you want a detailed aerodynamic analysis of a specific turbine situation then maybe ask on our [physics.se] site. There are many different kinds of turbine, and it's more than likely that some will spin slower and some fast with more blades.

The windmilling prop extracts energy from the mass flow through the propeller disk area. An 8-bladed prop can extract more energy from the mass flow, and does this at lower RPM than the 2-bladed prop. The extra energy it can harvest is due to lower tip losses.

Pic above is from Principles of Helicopter Aerodynamics by J. Gordon Leishman, and shows results from a combination of momentum and blade element theories (BEMT). With no viscous or tip losses to consider:

Notice that increasing the number of blades (or increasing solidity) does not affect the maximum efficiency of the turbine, but it does affect the tip speed ratio (or wind speed) where maximum efficiency is obtained. Too many blades ...gives a steep power curve that peaks at low tip speed ratios. This is not very useful in practice, besides the fact that the blades will also be approaching stall at these conditions (high wind speed).

So yes, a propeller with more blades turns slower when windmilling, because it has absorbed the maximum power at a lower rpm than the 2-bladed propeller. It is important to realise that the higher blade propeller turns slower by design - the blade angle is not fixed, but set according to wind speed, in such a way that the design tip speed ratio @ optimum is reached. And this is also the case with a windmilling propeller on an aeroplane, in fact the blade angle is set such that the prop does not turn.
In helicopter rotor measurements it has been shown that rotor solidity is a main design parameter: how much of the disk area is covered by blade area. How the blade area is distributed over the number of blades is of secondary importance, so a twin blade rotor performs much the same as a four-blade rotor with half the blade chord. Except for the noise and vibrations generated.

You can add explanation what is aerodynamic reason why more blades slow RPM down, something must happened with local airflow over blades if RPM is lower...

It is not true that 8 blade can extract more energy from 2 blades, it just do it at lower RPM..second sentence need correct that..

The picture is from theory without viscocity and tip loss effects, reality is different.

Your answer dont explain my main question.

Are these turbines with different numbers of blades using otherwise-identical designs for each blade?  I'm not clear whether the premise of the question is actually true for an all-else-equal comparison, e.g. take an 8-blade turbine and knock off 6 of the blades, then operate it in the same wind speed as before.  (With no load?).  vs. comparing two differently-designed turbines, designed to run with 2 vs. 8 blades in the same wind speed with some load, @JurgenM.

@PeterCordes Yes all blades have equal geometry. We must simplified question. also two propellers design  for same wind speed and RPM must have same twist..

It seems to me that a freely windmilling prop (not driving a generator) would have different characteristics than a windmilling prop driving a generator.  I'm not sure which case is being addressed here.  If we are driving a generator, I'm having trouble with the idea that adding more blades won't allow us to do more work-- if that's what is being suggested here--

This answer would be correct if it mentioned that the slower 8-blade turbine is geared differently than the faster 2-blade turbine. This is possible because the 8-blade prop produces more torque for any given RPM in its effective speed range than a two-blade prop at the same RPM. The prop is spinning slower, but the generator may be spinning at the same speed as it would be with a faster, two-blade prop geared differently.

@dotancohen Read again what is my question.. Everbody seems to prove my state that more blades rotate slower instead answer the question..

I read your question. For a given subsonic airspeed, with a given load (the generator in the RAT), two propellers with identical blades that differ only in the quantity of blades, will see the more-bladed prop spin faster. Thus the RAT will generate more power. However, by changing the generator gearing to take advantage of the higher torque the more-bladed propeller can produce, the resulting system can spin slower yet produce the same or even higher electrical output power at the same airspeed.

What is aerodynamic reason why turbine with 8 blades will rotate slower then with 2 blades?

It won't, all else being equal.
The speed of the turbine is governed by the aerodynamic forces on the blades and the resistance in the turbine. (By resistance I mean non-aerodynamic forces like drag in the bearings and the actual generator.) In a two-bladed turbine (or any other) the normal speed is where the total of the aerodynamic forces and the resistance balance.
If we quadruple the number of blades, making each of the added blades identical with the originals, then according to naive, oversimplified, aerodynamics we are quadrupling the aerodynamic forces on the turbine (the ones driving it). The resistance is not quadrupling, so the turbine will spin faster.
It won't spin 4 times as fast for two reasons. Firstly, as the turbine speed increases the non-aerodynamic resistance increases, and they will balance at significantly less than four times the speed. Secondly the aerodynamic behaviour changes as the speed increases, including both an increase in the aerodynamic drag of each blade and aerodynamic interference between the blades, reducing their "lift". (Aerodynamic interference is why biplanes don't have twice the lift of monoplanes. It can actually reduce the many-bladed turbine's aerodynamic force below that of the few-bladed one.) That's what I meant above by saying that we are quadrupling the aerodynamic force only for naive, oversimplified aerodynamics. These two factors means the 8 bladed turbine will top out at well less than 4 times the speed of the two bladed.
It's also important to note that 8-bladed fans aren't designed by simply taking a two-bladed fan and adding six more identical blades. The blades will be designed differently. Turbines have an intended operating speed and the blades will be designed so the turbine operates at that speed for expected airflows. Higher turbine speed does not always mean more power - a multi-bladed turbine may be designed to run at a lower speed and generate the same power as a differently-designed two-bladed one.

Your answer imply that using more blades means get more energy for given turbine diameter, that is not case in wind turbine theory. if this is case all wind turbine  will have more blades not just 3 or 2...

Actually no I'm not. That's only according to the "naive, oversimplified" aerodynamics, and I explain why not in the paragraph after. Also there are other reasons to not increase the number of blades, or to make fewer larger blades rather than more smaller ones.

@DJCalyworth This is very easy to test, just take two identical car ventilator, cut blades and leave one with 2 blades and one leave with 8 blades. Put of  window and meassure RPM...

Imagine a propeller being turned by an engine suddenly losing power.  How to make it a good generator as well as a good propeller?  Probably a more symmetric airfoil.  As seen here, a good airfoil shape will help create more lift/thrust from the relative wind.
Interference will play a role in rotation speed.  This is why more blades have a lower maximum rotation speed.


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Why does lift decrease and drag increase when approaching the stall condition?
This is an ECQB-PPL question specifically asking what happens before the stall. I do understand that after the critical angle has been reached, the airfoil is no longer generating lift (so obviously the lift decreases) and parasitic drag increases dramatically. However, it is not clear to me why this is true when approaching the stall condition.
The magnitude of the lift vector is described by the following formula and $C_{lift}(\alpha)$ still increases (albeit slowly) when approaching the stall condition $\alpha_s$:
$$
L = \frac{1}{2} C_{lift}(\alpha) \rho V^2 S
$$
Or is it a purely geometrical consideration, and what is meant by this question is that as the lift vector is assumed to be perpendicular to the airfoil chord average local relative airflow, its vertical projection (that should counteract weight) decreases, and its horizontal projection adds to the drag?
Just to highlight the reason why I'm confused: my understanding of the definition of "stall" is that $C_L$ has flattened out and maximum lift has been reached. Therefore, I thought that the lift would still (slowly) increase, and not decrease when approaching the stall (but not having reached it yet).
This is assuming that the airplane is otherwise in a stable level flight.

Who ever said that lift decreases while approaching a stall? That sounds incorrect to me.

This is an exam question from the current ECQB-PPL 21.2 set (E51-200511-00068). Specifically, it asks "How do lift and drag change when approaching a stall condition?" and the accepted answer is "Lift decreases and drag increases".

Anyways in my opinion approaching it means when you reach.

Stall point is defined as the peak in the $C_L$-$\alpha$ curve. So when approaching stall, the rate of increase reduces, bit $C_L$ still increases.

The question should be why does coefficient of lift decrease when approaching stall. Once example is a high g turn or loop, where the lift increases with g force acceleration while approaching but not yet in a stalled state. If the angle of attack is excessive, typically one wing stalls before the other, resulting in a snap roll.

Related: What is the fundamental physics behind L/D decreasing as AoA increases beyond the optimal point?

Both Lift and Drag increase as AOA increases, but as Lift is the normal (to the flight path) component of all aerodynamic forces, and Drag is the parallel component, because the total force is tilting further and further aft, the parallel component (Drag) increases faster. Eventually, at some point, when AOA has increased sufficiently that the total force vector is pointed more aft than some threshold angle, or when boundary separation begins, Lift will begin to decrease as AOA continues to increase. This is simple vector mechanics, or geometry.

Analogously, if you attach a string to a weight attached to a vertical pole that it must remain attached to, and stand 20 feet away, and pull, the weight will not rise until you pull with a force much greater that the weight. If you stand one foot away, you will find the task significantly easier. This is the same basic vector mechanics.

A simple and arguably easy to understand answer is that the wing does not produce separate forces called Lift and Drag. It produces force at every point of the wing *, pointed in a direction normal (perpendicular) to the surface of the wing at each point. Lift is just the sum of the the components of all these forces that are normal or perpendicular to the flight path of the airplane (it's direction of flight through the air), and Drag is just the components of all these forces that are parallel to the flight path. (See image in @Koyovis answer above ... All those little graphics attached to the upper surface of the wing exists everywhere - at every single point on the surface of the entire airframe), and every single one of them produces a Lift component, and a Drag component.
So, as an airfoil approaches a stall condition, it's Angle of Attack (AOA), is increasing, i.e., the angle of the wing with respect to the flight path is getting steeper, so every one of those actual real forces acting at each point of the airfoil is tilting more and more aft (to the rear). Therefore the component of those forces which is perpendicular to the flight path (LIFT) is decreasing, and the component parallel to the flight path (DRAG) is increasing.

Aerodynamic forces are just the static pressure of the air pushing on the surface of any airfoil (or, in fact, any shape) moving through the air. These forces are caused by the collisions of air molecules hitting the surface and rebounding. In these collisions, momentum is exchanged, and the aerodynamic force is just the result of the change in momentum. The formula is just F=ma.

This formula is actually a shortcut for F=dP/dt. Where P is the momentum (Mass x Velocity or mV), so dP is the change in momentum, and dP/dt is the rate of change of the Momentum. It is the same as F=ma because a is the rate of change of Velocity (dV/dt) so F=ma  is the same as F=m x dV/dt which is the same as F = d(mV)/dt, which is the same as F=dP/dt since P = mV

Thank you everybody for your answers and sorry for the delay, I was really busy with the exams! It's hard to choose the "right" answer, but I guess this answer explains it best in the spirit of ECQB and confirms my suspicion, that they basically meant that (assuming that the airflow is horizontal), the vertical component decreases and the horizontal component increases as the lift vector rotates (even if the magnitude of "lift" itself still increases until $\alpha_s$)...

@Yuri, I might add the trivial correction to your parenthetical comment about the flow being horizontal. It doesn't matter whether it is horizontal, Instead of making that assumption, all your concepts and statements are equally true if you think about things from the frame of reference of the aircraft, and it's flight path through the air (opposite the relative wind), and not of the ground. The Lift is then the component of the total force perpendicular to the flight path, and the drag is the component parallel with the flight path.

Of course, you are right - I was just using terms like "horizontal" and "vertical" as the question pertained to straight and level flight, and it's easy to imagine and explain. Thank you for the elaboration!

Yes, sir, right on!

After the stall is reached the airfoil still generates lift, but less than before the stall. Plus drag has greatly increased due to the separated boundary layer on top of the wing, which causes pressure drag.
Image source
Figure above shows why the boundary layer separates: the airflow close to the skin is slowed down by friction, and does not have enough kinetic energy to overcome the pressure gradient along the upper aft side. Static pressure in the Wake region of the picture is quite low, causing the pressure drag.
Loss of lift is also because of the separated airflow, the flow aft of the separation point is lost for lift generation. Lift loss is not due to wing tilt: the lift vector is perpendicular to the airspeed vector, not to the wing chord.
Image source @ Re=10$^6$
Above process takes place gradually: $C_L$ increases linearly with angle of attack until the separation point is located at the wing aft tip (about $\alpha$ = 12° in the plot), then gradually diminishes with AoA until it peaks out at $\alpha$ = 16°. At this max $C_L$ peak there is quite some separated airflow at the top wing area already.
Looking at the example above, the point at $\alpha$ = 12° would be the point of stall onset, where the lift coefficient starts to reduce and turbulence may be detected but lift still increases with AoA. The point at $\alpha$ = 16° is the stall point: further increase of AoA results in a reduction of lift.

I see, so it's really the magnitude of the lift vector that decreases, and the main contributing factor when approaching the stall condition is $S$, being strongly reduced by growing separation, whereas slightly increasing $C_L$ and slightly decreasing $V^2$ do not play an important role here, correct?

It is the magnitude indeed. Approaching stall, as in no flow separation yet, means the airfoil is not yet stalled. It is the flow separation that causes non-linear changes. S does not change, it is the projected wing area.

Sorry, unfortunately, I still don't understand your answer :-( Then why does the magnitude of the lift vector decrease before reaching the stall (as in the original question)?

Have edited the answer. Magnitude of the lift vector peaks at the point of stall, by definition.

The answer start by understanding how are aerodynamic forces generated. The aerodynamic forces are a result of the pressure distribution around the airfoil and of the shear forces due to the viscosity (look at the wikipedia definition, without entering too much into details).

Starting from this, we can have a look to a simple case (NACA0012) in which we have a symmetric airfoil at 3 degrees. The flow field result to be "attached" to the airfoil and we have the maximum efficiency, because we do not have "ricirculation zones" (mathematically we can pose the Hypothesis of Potential flow and say we have no zone with vorticity different from 0, if you want to know more have a look at Helmoltz' theorems). The more we increase the angle the more we happen to have an adverse gradient in the back of the upper face of the airfoil. At a certain point that adverse gradient brings the boundary layer to separate and the flow being no more attached (have a look at this to understand the mechanism). This bring to a different pressure distribution as you can see from the picture of Thwaites I attach to you here.

As you can see the resulting of the pressure distribution (the area under the line) is less prominent. This can be easily described with the Bernoulli theorem. You can imagine that the fluid vane is limited by the de-attached flow and hence you have an increase of pressure with respect to the attached flow. So finally this is the physics behind the stall and the degradation of the aerodynamic performances (abrupt decrease in lift and increase in drag, due to the form drag).
Regarding this exists different type of stalls. To know a bit more, I suggest you to read this NASA report.
The fact that the lift decreases before the stall depends on the type of stall that you have. On the ECQB I think that they mean with "stall" not the degradation of aerodynamic coefficient stall but the case in which the flow is completely separated. There are stalls in which a laminar separation bubble is placed on the upper faces, but you have a re-attachement due to turbulent transition. You can see an example in the figure below (leading edge short bubble).

Here you have a decrease of the lift before the complete separation of the flow, hence it would give an answer to the ECQB question.

Thank you, but it seems that like @Koyovis you are explaining what happens after (or at) the stall (abrupt decrease in lift and increase in drag), but my question is why in ECQB they claim that lift decreases before reaching the stall.

Edited the answer to try to address the specific question.

At airfoiltools.com you can examine many relationships of lift and drag without worrying about the "right" answer on a test.
In one particular course of study, "approaching stall" may be the warning buffet of airflow separation from the trailing edge of the wing.  With a magnifying glass, you may see lift begin to decrease here as drag continues to increase.  Maybe in a range of a few tenths of a degree AoA, before the wing fully stalls.
But, as the "polar graphs" indicate, and as a pilot, the exact AoA this happens is trivial, and different airfoils can have vastly different behaviors near stall.  Is is best to use V$^2$ for adequate lift when flying and stay away from excessive AoA.
A possible theoretical explanation for a certain wing may be within the 3D realm of examination, that being the wing tip vortices begin to curl back into the top of the wingtip more at higher AoA, thereby reducing total lift just before stall.
One might speak to their professor regarding that subject.


text:
Package Substation below the VORTAC Counterpoise
Is it recommended to install a 100 KVA 13,800/400 Volts, 3 Phase,  60 Hertz Package Substation beneath the VORTAC Counterpoise Metallic Grids and what will be the effects if the substation is already energized with the VORTAC also in operation?

This seems like the kind of question that should really be asked of an engineering consulting company, most likely an electrical engineering firm with experience in RF analysis and shielding and, ideally, aviation and its associated regulations—not a bunch of Internet randos.

No. This is a very bad idea, for the following reasons.
First of all placing a very large chunk of metal under the counterpoise grid will affect the uniformity of the radiation pattern of both the unidirectional and the omnidirectional signals being transmitted by the VOR station.
Secondly there is a risk of coupling RF currents into the substation and the power leads coming into and out of it. This may upset the switching and regulation hardware inside the substation, as well as turning the power leads into antennas, further disrupting the pattern uniformity of the transmitted signals.
Part of the test & qualification process for such a construction project will be to collect RF field uniformity data in the complete 3D space surrounding the VOR all the way out to its maximum intended range in the complete absence of the substation, repeating the test with the substation under the grid but with power disconnected, and once again, with power connected.
Note that there are both commercially-available and freeware antenna simulation software packages available with which the effects of the substation on the VOR signals can be predicted. I recommend that you halt the construction project until after you have run the models and proven to the aeronautics authority that the substation will not perturb the VOR an any way.


text:
Why do gate agents wait to ask for volunteers minutes before departure?
E.g., taking the last flight out of the day. It checks in full, and the airline knows this several hours in advance.  Getting bumped would mean spending the night here again. Would be nice to avoid the commute to and from the airport, and the double dose of TSA. Why doesn't the airline text the pax and ask for volunteers via an online auction?

I'm voting to close this question as off-topic because it belongs on Travel.SE.  The passenger travel aspects of aviation are specifically off-topic here.

Flights are oversold (on many airlines, tho not all) because it generally works - somebody doesn't show up (takes a different flight, oversleeps, trip cancels, etc) and what was 163 seats sold for a 160 seat airplane, is now 160 or 159 people wanting to travel. But sometimes, everybody shows up, and they have to ask for volunteers. But it's not until everybody HAS showed up, that you know how much you're really over. Nobody PLANS to oversleep or have a late change of plans, but it happens. It's only when you know that you really DO have the 163 people present, that you ask for 3 volunteers.

@RalphJ, actually, if you sell 163 seats, you can reasonably expect 148 passengers to show up.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
Is this photo showing dozens of airliners in close proximity real, or even plausible?
CNN has an article about Heathrow that's accompanied by the following photo:

Is this photo depicting an actual event?  If not, would it even be possible/plausible to fly this many airliners this closely together or would wake turbulence and similar factors make this basically impossible?
It looks like several of the planes may be in the process of either stowing or deploying their landing gear, and also have their flaps deployed.  Does this indicate that maybe someone has photoshopped together a bunch of photos of planes landing to produce this image?

It is not real, but it may be a time-lapse.

Here’s a link to similar work: https://www.mpkelley.com/projects/ I do not know whether the picture posted in the question is part of that series or is by the same artist.

Tips for finding information or even the soucre of images: 1) tineye.com finds other instances of a particular picture 2) Look at the image credit/byline (CNN curiously did not have that for this picture, and search for that 3) right-click the picture and select "Inspect element". In this case you find text that says "Heathrow Airport composite photo".

The image is a composite image of 42 planes taking off from Heathrow over a one-hour period on November 2, 2016.  Here it is on Getty Images if you want to license it for your own purposes;  it is credited to Dan Kitwood.

Excellent sleuthing!

It’s a composite shot of multiple departures from an airport.  To answer the question, no, there was no single event with all of these aircraft flying in close proximity like this where a single photograph of the group could be taken.

It's a photoshop done for artistic purposes.

It is a Photoshop image and not done too skillfully at that, may be on purpose.

The paths are not parallel, all tracing back to a shared vanishing point. They are on a collision path.
The perspective geometry is not right, meaning an object depending on the distance and position in a camera field distorts in a way that the points and lines closer to camera get blown up in scale and the lines and geometry further back gets flatten a bit. This is the reason faces get distorted in selfies with nose getting out of proportion.
Even in the event of an aerobatic airshow where the planes trajectory explodes open to a flower pattern or spills into a cascading dive these perspective rules are valid and in fact taken advantage in photos and videos of the event.

@egid sorry, I will edit my answer.


text:
How does an FBO refer to the parking spots on the ramp?
How does an FBO refer to the parking spots on the ramp? What is the official term? And is there some sort of an identifier, a "ramp number"? Is this standardized, like runway numbers, or does it vary from FBO to FBO? Are there numbers painted on the ramp?

From the airports I've flown into, it's usually, you're just guided into a spot, and most often, there aren't any lines there, mainly since an FBO should be able to generally serve something as small as a Cessna, to a Gulfstream, or bigger. Flight School Ramps might have painted lines and dedicated parking for their fleet, but they know who will be parking there and what type.

You are right.  Usually it is "park it next to the Mooney at the end of the line, over there

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
Do multi-engine jets need all engines with equal age to reduce asymmetry in thrust and fuel consumption arising out of deterioration?
Let's assume, one of the engines on a twin-jet needs replacement and the jet is is fitted with an old engine and a newer engine after the due replacement, the older engine would tend to produce less thrust at higher fuel consumption due to deterioration on account of aging compared to the new one. Though the aircraft are designed to handle such asymmetries and modern FBW FCS should be able to handle it without bothering the pilot for any manual corrections, logically it makes sense to think one would like to do some basic engine matching to make sure the asymmetry is as small as possible. Do they follow this in real life? Are there guidelines in MRO procedures specified (both Civil and Military Jets) on how to choose a replacement engine, so as to maintain symmetry in the thrust profile?

A twin-jet should be able to fly and land with only one engine. This maximal thrust asymmetry should be tolerated too.

@Uwe I mentioned the same in the question itself that the Jets are designed to handle such asymmetries.

Yes, and pilots should be too.  Good point Uwe!

It doesn't quite work that way.  When an engine wears out it's rated thrust doesn't decline; its "ITT Margin" (or some similar phrase - basically its thermodynamic margin at takeoff) declines.
When setting takeoff thrust on a turbofan there is a target N1, or target torque on a turboprop or turboshaft, or Engine Pressure Ratio on a pure jet, and the engine has to achieve that target, so it's making rated thrust or power, more or less, no matter how old it is.  What happens when it ages is the temperature required to get there goes up.
There is a maximum hot section temperature  (Inter-Turbine Temperature - ITT or Turbine Inlet Temperature - TIT, or Turbine Outlet Temperature - TOT, or something else, depending on where the probe is) for the engine when producing takeoff thrust or power.  On a new engine, the actual maximum temperature that you will see will be well below the maximum allowable (the temperature margin).
As the engine wears (turbine and compressor blade erosion mostly) and its efficiency declines, what happens is the hot section temperature when the engine is at takeoff rating goes up.  It creeps higher and higher as the engine ages until eventually the margin is gone (or down to some minimum), temperature exceedences on takeoff may start to occur, and it's time to come off.
So there isn't really a thrust asymmetry problem with mixing new and old engines, although there will be a small fuel flow difference, but you have an automatic balancing system to account for this.
Staggering engine times is common.  On an older, heavily depreciated jet especially, the engines may represent a huge part of the value of the airframe and it's a common practice to stagger the age of the engines avoid having to change out two at once when they wear out.

That's also good for reliability.  With two matching engines with identical use and maintenance history, if one fails, the other one has a chance of failing for the same reason, especially when you increase stress on it due to it being your only remaining engine.

@john Thanks for the reply. makes sense for the Civil jets. But what about Mil jets..? I have read somewhere and also heard from Air Force Maintenance personnel that the Mil jets suffer thrust loss which could be as high as 15-20% as they come to end of life. Even Overhauls cannot recover the loss of thrust that happens between the two overhaul. I am thinking the loss of thrust is because Mil jets run at max TET possible right from starting without any margin left to cover deterioration. So any thrust shortfall cannot be compensated by increasing TET by the FADEC.

@jayS Mil jets are completely different. You need to actively prepare them for flight.  There's a reason why a hurricane destroyed so many grounded F-22s; they weren't made ready to fly.

Another interesting thing that happens is upgraded engine models will come out with increased thrust ratings, with the same core and higher hot temperatures, but without a max ITT increase.  Effectively this shortens its service life.  The CF-34-8C5A1 used on the CRJ1000 was affected by this.  Operators were strongly encouraged to use flex thrust.

N1 limits are often a factor as well, as compressor erosion takes place.


text:
Why are the Boeing 737 nacelles odd shaped?
Why is a Boeing  737 NG engine not completely round in shape? It seems to be flat at the bottom and round at the top and sides. Earlier Boeing 737 versions (100 and 200) seem to have a more rounded shape.
(Source)

Thanks to all of you who took time to clearly explain the reasons for the shape of the nacelle/engine in Boeing 737NG. Srikanth

I think if you look at the image with some imagination, it should be obvious that the shape has advantages in crosswind landings.

The engine in question is a CFM56 turbofan engine which is larger than the JT8D the plane was originally designed for and thus has less ground clearance. This meant that they needed to flatten the bottom by moving the accessory gearbox to the side from the bottom and shrinking the fan.
To quote Wikipedia:

In the early 1980s Boeing selected the CFM56-3 to exclusively power
  the latest Boeing 737 variant, the 737-300. The 737 wings were closer
  to the ground than previous applications for the CFM56, necessitating
  several modifications to the engine. The fan diameter was reduced,
  which reduced the bypass ratio, and the engine accessory gearbox was
  moved from the bottom of the engine (the 6 o'clock position) to the 9
  o'clock position, giving the engine nacelle its distinctive
  flat-bottomed shape. The overall thrust was also reduced, from 24,000
  to 20,000 lbf (107 to 89 kN), mostly due to the reduction in bypass
  ratio.

They also mounted the engine in front of the wing rather than below to get every bit of clearance they could.

I added three pictures, if you disagree with it, please feel free to remove

the LEAP's fan blades are 173cm in diameter

Gotcha, sorry about that. I'm editing this answer to remove a rendering of the wrong aircraft/engine.

To add, the slope visible on the left picture at the bottom of the nacelle is there to allow the aircraft to land on one rear wheel during, for example, a crosswind landing.

You are confusing the shape of the engine with the shape of the nacelle. Axial-compressor jet engines (standard on larger commercial jets) are invariably round and thin1. Engines with a centrifugal compressor are fatter in the middle and will occasionally be lumpy shapes depending on the internal airflow (some engines completely reverse the flow of air through them, sometimes several times2).
Positioning accessories like pumps, starters, generators, control boxes and minor things like mounting brackets are flexible. The engine manufacturer may dictate the position of some things like a gearbox, but if it attaches with a hose it can generally go anywhere the airframe designer wants to put it. The nacelle is then built to contain everything, not produce too much drag, not hit the ground, and preferably not cost too much to make. There's no reason we can't make hexagonal nacelles, we just choose not to.

I consider the fan to be a bolt-on here, not part of the engine itself.
See Pratt & Whitney PT-6 for an example.

This question seems to have been worked over pretty well. My only additional comment is some background on the engine itself. Think of the SNECMA CFM-56-3 Turbofan as two parts: 1) the core gas generator designed and built by GE, and 2) the Fan/Power Turbine designed in France and mated onto the GE core. The best part of the story (you will not find this in Wikipedia) is the Boeing redesign of the engine strut to allow this engine to be mounted in front of the wing as opposed to under the wing such as with the -100 and -200 installations.
Another point that seems to be missing is FOD. During turbine engine design, considerable time and money is invested in designing features that protect the turbo machinery from FOD (Foreign Object Damage). This engine, if it was any closer to the ground (such as during operation with a deflated landing gear strut) turns into the most vicious vacuum cleaner in the world and sucks up everything in its path.

The "in front of the wing" vs "below the wing" design in the 737 -300 and onwards is also in contrast to the "below the wing" design of the A-320 series, which use the same motor but need a longer landing gear to accommodate the way the engine is mounted.

Is there any advantage of having a shorter landing gear as in Boeing in comparison to the Airbus planes of similar size..?..

@Srikanth: The original 737 was designed (in the mid-1960s) to sit low enough to allow baggage to easily be loaded by hand without the use of machinery.  The 737 Classic (the first series with the hamster-pouch nacelles) and the A320 were both designed much later (in the early 1980s), by which time baggage-handling machinery had become much more common than it was back in 1967, but the 737 Classic stuck with the original low ground clearance to allow it to be certificated as a derivative of the original 737; the A320, being a totally new design, couldn't take advantage of this, so Airbus (1/2)

went with the longer landing gear to further reduce FOD risk (and, for that matter, tailstrike risk) and allow the use of a larger, higher-bypass, more efficient version of the CFM56.  (2/2)


text:
Stop flight training and start again in a different country
Is it possible to put PPL flight training (started in the US) on hold and pick it up again in a different (European) country? Would one have to start all over again or would the US training be recognized as valid?
I understand it's a pandora box question with many details under the hood but I was wondering if anyone has ever experienced something similar and can share their thoughts.


text:
Can I ask ATC a radio check with a handheld radio?
I bought a handheld portable radio capable of transmitting in the air band and I was wondering if I can request a radio check to the ATC to see if the radio works properly. I know it's illegal to talk over those frequencies unless you're an aircraft (or at least I suppose it is), but I wonder if it's ok in this case.
If it's ok to use that channel just to test the radio, what's the best way to go about it? Just "Radio check"? Or "<Airport> tower/ground, radio check"? Or should I acknowledge the fact that I'm not an aircraft with something like "<Airport> tower/ground, on the ground with a portable radio, radio check"? Thanks

Do you have a radio license?

No I don't. To add more color, I'm a student pilot so I do plan to use my radio legally (if necessary).

Please indicate what jurisdiction you are in. The radio rules are quite different between different countries.

I'd add that even with a (ham) radio license, most if not all countries would consider this illegal. You're free to receive any frequency, but to transmit in certain frequency bands, you need a proper license for that frequency.

@jayded-bee Receiving any frequency is illegal in many countries. Even receiving air-band is illegal by default in many countries.

This is in the US.

@user253751 Nobody can stop you from listening to the EM waves traveling through the air, because it is impossible to know whenever somebody is receiving the signal. Reverse-engineering encryption and thus snooping on data that is not intended for you may be illegal, but that's a different issue. Receiving is not traceable.

@jayded-bee there is a difference between "legal" and "illegal but they probably won't catch you". Look what the British government did to Alan Turing.

@user253751 If owning a radio station is illegal where they live then OP has other problems

Clearly the answer is yes. Whether or not the regulations specifically allow or prohibit it is another matter altogether. It's unlikely that you'd ever make it to the airport without violating the letter of some local, state, or federal regulation - there are just too many.

@jayded-bee Note that I said "receiving any frequency" and not "owning a radio station". It's hard to see your comment as anything other than bad faith.

@user253751 how is commenting on an internet forum "bad faith"? I don't see how it could be legal to own a radio station capable of receiving radio signals yet illegal to actually receive them? That is completely unenforceable. I'd like to know which countries have inane laws like this.

@jayded-bee because I said "it's illegal to receive any frequency" and you decided to interpret that as "it's illegal to own a radio station"

@user253751 yes, because that was the logical conclusion for me, as explained above. What strikes me as odd is the continued claim that such laws exist somewhee but not a single example; I could say the same of your comments, but I'm more interested in learning than in calling out my assumptions of other people's intentions. Take that as you will, I don't consider users on here to be boogeymen.

@jayded-bee same laws as https://en.wikipedia.org/wiki/Radio_scanner#Legislation

I think there's a technicality here.  Both the operator and the radio station each need, independently, an FCC license.  The operator license requirement has been waived for pilots for a long time, and in 1996 the requirement for the plane to have a station license was dropped so long as the airplane is in US airspace.  When you operate a handheld in the plane, you are operating a second radio within the same station (the airplane) so you are covered on both counts.
The problem is outside the plane you are not operating in a licensed or license-waived station, so you would need a station license for the radio, AND you'd need a restricted radio-telephone operator's permit because you don't fall under the pilot waiver because you're not flying a plane.
Virtually ANY ATC facility will not know any of this and will almost certainly give you a radio check.  It would be a very rare circumstance that anyone would call you out.  I wouldn't do it on clearance delivery at O'Hare, but elsewhere it's unlikely anyone will care.  Unlikely they'd even ask for a tail number if you said "Rochester Ground, how do you read?"
But as the previous poster points out, it is technically illegal and can have consequences.  I have both FCC and FAA licenses and I too have heard of people being convicted for transmitting on public service frequencies.  So I'll clarify...  If you are a pilot, AND you are at the airport, and you are making a legitimate radio check, it is technically prohibited but you would have some latitude that a layperson would not.  But if you are not a licensed pilot to begin with, it's probably not the best idea.

The key is in a comment to the original question:

I'm a student pilot so I do plan to use my radio legally (if necessary).

which leads to two possible answers:
1 - Talk to your instructor. They should be familiar with procedures for handling this.
2 - Use it on a plane. Bring it with you on a flight. Then test it from inside the plane, which should be legitimate as you are then testing it as a backup radio for legitimate use, and can actually identify yourself as a pilot of an actual aircraft if questioned.

Test it in the plane, while on ground, just to avoid any potential issues, like student task saturation, interference due to faulty equipment or any of the potentially complicating factors not needed when airborne.

Yes. You will be just fine. I’ve done it many many times before.

Can you elaborate in your answer? E.g., in what jurisdiction were you in? What licenses did you have? What were some (example) circumstances (at home? at an airport? Time of day? Purpose of the test? Etc.)? Distance to the ATC tower? What was the transmit power level?

This is in the US.  I can’t imagine any controller else where in the world would have such a stick up his @ss as to be annoyed by such a request.  This is being done insure that one can establish two way communications, and only a real idiot/goon would complain about such a thing.

@CarloFelicione You should [edit] your answer to include this information, rather than separating it from your answer in a comment.

Are we trying to set a record for just how pedantic and puerile we can be here in the comments? It's ridiculous, and reflects poorly on our community.

Ground stations have to be licensed to transmit on ATC frequencies. To get a license you will need to have a legitimate reason for the use of the radio. Until then, do not transmit on ATC frequencies. During my career, a man got a year in federal prison for the unauthorized use of a portable radio. It is considered interference with the ATC system.

Was he a pilot doing legitimate aviation-related stuff? Or some rando interfering with aviation safety?

Rando interference

Well, there you go.

@RetiredATC by requesting spurious radio checks?


text:
Can the TSA require supplemental screening AFTER a flight has landed?
A few days ago, Kahler Nygard reportedly received a boarding pass indicating secondary screening was required, but that pat-down was reportedly never done. So when the plane landed, they took the passenger off the plane ahead of the others and requested that he submit to the screening at the destination airport, after the flight. He recorded the incident on his phone, and after confirming that he wasn't being detained, simply walked away.
The proposition that screening a passenger after the flight has landed might retroactively increase the fight's safety is, I think we can all agree, counter to the fundamental nature of causality.
But the question is this: Is screening passengers after they have arrived at their destination without incident within the rights and responsibilities of the TSA? Clearly it doesn't jive with the whole "cause and effect" framework of reality that we all live in, but are they within their rights to demand such screening anyway?

The TSA can screen people when they like, even after a flight. They don't usually do it after because it's pointless. It's probably an example of CYA.

Not necessarily pointless. While the TSA is supposed to protect flights, they are also one part of the whole Dept of Homeland Security and are also chartered with protecting the entire US. If there was a security concern about the individual (I am not stating there is/was such a concern as I have no information regarding this specific incident at all), it's the TSA's duty to perform whatever checks are needed to ensure the safety of all US Citizens. (I'm also not stating whether I, personally, agree with what the TSA does, or not).

The answer is contained in the question. The TSA can ask to search someone after the flight, but the don't have to submit. It's stated in the account.

This question is probably better suited on Travel.SE, since it pertains to airport-side procedures

TSA staff actually have very little authority - they can prevent you boarding an airplane, they can stop the airplane from leaving. For everything else, they call the local police (the phone is their Big Stick - they can call in some serious assistance, like SWAT or some loaded F-22s).
This sounds like a classic bureaucratic whoopsie - someone missed the initial screening note, someone else noticed that the box wasn't ticked in the system, and protecting the monthly performance statistics prevailed over common sense. Since the passenger declined the search they can prevent said passenger from boarding a flight, but since he did not wish to board a flight (and has not done anything illegal) all they can do is ask him to leave the secure area. As that is exactly what the passenger wanted to do anyway.....

their big stick is people thinking they have the right to do whatever they like... And of course the fact that that includes the cops and other security agencies they can call upon who will do whatever the TSA asks them to without thinking because it's after all DHS business and everyone cooperates with DHS else you're in trouble.


text:
Is ILS secured from being hacked/overriden/disturbed?
Since ILS uses radio frequency. Then is it possible that some one transmit a malicious signal on the same frequency ? Are there any measures to prevent that ?


text:
What formula to use for calculating pressure altitude?
I'm having some trouble making my calculations of pressure altitude match those in apps and text books.
I'm using the formula from Wikipedia:

(29.92 - altimeter setting) * 1000 + airport elevation

In the app I'm using, the altimeter setting is currently reported as 29.70 with an airport elevation of 22. Using the above formula I would assume:

(29.92 - 29.70) * 1000 + 22 = 242

Yet the app is telling me it is 229
Even more confusing, when looking at the Airman Knowledge Testing Supplement for Sport Pilot, Recreational Pilot, Remote Pilot, and Private Pilot (FAA-CT-8080-2H), figure 8, page 2-8, the pressure altitude for altimeter 29.7 is listed as 205. I assume this is using an airport elevation of 0, but then I would expect the answer to be:

(29.92 - 29.7) * 1000 + 0 = 220

So my questions are:

Which answer is correct? Wikipedia, the app or appendix 2?
Why are there 3 different answers?

The formula from wikipedia is a rough estimation. Think of it as a rule of thumb. It's useful if you need numbers for rough calculations. However, the exact formula is as shown below:

According to that formula, I calculated pressure altitude difference for 28.92 and it's 938 feet, as opposed to 1000 feet. Additionally, as you can tell from the formula, the relationship is not a constant linear value. So difference of 1 inch of HG is not always 938 feet, it's changing.
So, the FAA guide is accurate. And I would say your app is accurate enough also, however probably omitting some numbers after decimal, hence the small difference. Of course, you need to add field elevation on top of those numbers as well.

The formula you show is not for computing the pressure altitude, but to compute the pressure at a given altitude in the standard atmosphere

Given an altimeter setting ($\mathrm{QNH}$) and the elevation of the airport $h$, the exact equation to compute the pressure altitude is:
$$\mathrm{PA}=h+\frac{T_0}{L}\cdot\left[1-\left(\frac{\mathrm{QNH}}{P_0}\right)^\frac{R_s\cdot L}{g}\right]\tag{1}$$
where:

$h$: Elevation of the airport in meters
$L$: Temperature lapse $=0.0065 \mathrm{~K/m}$
$T_0$: Standard temperature $=288.15 \mathrm{~K}$
$P_0$: Standard pressure $=101325 \mathrm{~Pa}$
$QNH$: Altimeter setting $\left[\mathrm{~Pa}\right]$
$g$: Gravitational acceleration $\approx 9.81 \mathrm{~m/s}^2$
$R_s$: specific gas constant for dry air $\approx 287.058 \mathrm{~J \cdot kg^{−1}K^{−1}}$

With the above equation, a $\mathrm{QNH}$ of 29.70 corresponds to a pressure altitude of 204.95 ft. So I'd say the most precise is the book. Your app seems to be off by 2 feet (it should report 227 ft).

The well-known formula to compute the pressure altitude is just a rough estimation:
$$\mathrm{PA_{feet}} = (29.92 - \mathrm{QNH}_{inHg}) \cdot 1000 + h_{feet}\tag{2}$$
It can be derived from equation 1 via a Taylor approximation computed at $\mathrm{QNH} = P_0$:
$$\mathrm{PA}(\mathrm{QNH}) = \mathrm{PA}(P_0) + \frac{d\mathrm{PA}}{d\mathrm{QNH}}\cdot(\mathrm{QNH}-P_0)=\tag{3}$$
$$= h+\frac{T_0R_s}{gP_0}(P_0-\mathrm{QNH})=h+0.0832\cdot(P_0-\mathrm{QNH})$$
The constant $0.0832$ has dimensions of $[\frac{m}{Pa}]$ but if we convert $h$ in feet and $P_0$ and $\mathrm{QNH}$ in inHg we get:
$$\mathrm{PA_{feet}} = (29.92 - \mathrm{QNH}_{inHg}) \cdot 924.5 + h_{feet}\tag{4}$$
which should show where equation 2 comes from. Using 1000 instead of 924.5, beside making it easier to compute, also makes it more precise for values of $\mathrm{QNH}$ that are not too close to $P_0$


text:
If the speed of wind at ground is 200 kn, could an Airliner land vertically if it's course is opposite to the wind's direction?
I'm not a pilote and my field is programming, but I was thinking about that question:
If the speed of wind at ground is 200 kn (just an example), could an Airliner land vertically (per reference to ground) if it's course is opposite to the wind's direction?

Yes. Only if the plane is able to fly at 200 kt, and I'm not sure the pilot will be able to stop the engine and/or turn to exit the runway. Why the downvote?

Hi. I did not downvote, someone else did. Thanks for the answer and I get your point for exiting the runway.


text:
How should an aviation novice evaluate 'flight experience' or 'flight training' services?
Some companies offer a service wherein a complete novice may take a flight in a small aircraft, sometimes as a passenger, sometimes 'taking the controls' under the close supervision of an instructor. Examples of this include:

Spitfire Flight Experiences
Fly a MiG-29
Laser dogfighting

How should a novice, who has little or likely no flight experience at all, evaluate whether an offer such as this is:

Safe, within reasonable bounds (ideally the same safety precautions that a well-run national aviation regulator would require for full-time student pilots)
Legal, again within the remit of the regulations that would apply in that particular country
Reasonably likely to offer what is claimed i.e. where 'taking the controls' is advertised, that this would constitute actually manoeuvring the aircraft and that this would be legal and proper within the regulations

You are taking the controls as far as the instructor is willing to let you take them.  It's not like the pilot just sits there reading the paper while you do your thing.  He's probably making fine adjustments to control inputs while you make the large inputs to make the airplane poit where you want it to point (I do, for intro rides).

For Laser dogfighting, you linked to SkyCombatAce.
I did the Sky Combat Ace experience in Las Vegas about 4 years ago, and there were no lasers involved.  Rather, the back-seat instructor judged when you were in position for a kill shot, and just radio'd "kill", at which point the other plane activated smoke and started spinning.
Although I enjoyed my experience with them, its worth noting that they'd have 2 major crashes resulting in 4 deaths in about the last two years.   The investigations are still pending, but personally, I don't think it looks good.
In the April 2016 crash, the local weather was announcing thunderstorms in the area, and the 3 planes took off anyway, initially requesting flight directly towards the storms until the Tower Controller suggested they reconsider.  They either seemed oblivious to the local weather, or didn't care about it.  It has not yet been determined if that was a factor in the crash.
On the one hand, I enjoyed my flight.  On the other hand, I now have serious reservations about their operation, and would not consider flying with them again.

As far as the "taking the controls" aspect:  I had my Private Pilot's license at the time I flew with them, so I'm familiar with flying, although not at the performance levels of those planes.  However, "taking the controls" meant the ailerons and elevator only; no throttle and no rudder (controlled by the instructor).  In that sense, it was a lot like a simplistic video game with a joystick.

Update: July, 2018
The Factual Report on the first accident, in Henderson, NV is now available on the NTSB website.  The report took over two years from the incident to come out, which is atypical (6 months to 1 year is more common).
The report had two concerning findings:

First and foremost, the report finds the pilot had detectable levels of THC (marijuana) in his blood and tissue.  The report points out that it is impossible to tell how recently the pilot used it or if he was impaired at all. However, that he had any amount of an illegal substance (marijuana did not become legal in Nevada until several months after the accident) is deeply disturbing.

Secondly, the report calls into question if the flight operation is operated legally by CFR 14.91 and .61 (Instructional Flights), and suggests they may have been legally required to hold a part 119 air carrier / commercial operator certificate and operate under the stricter 121 or 135 rules.

I will reiterate that I enjoyed myself during my flight with SCA.  But after two fatal accidents and this incredibly concerning report, I cannot recommend that anyone fly with them until major legal and operational issues are resolved.

Update: May, 2021
A GoPro camera which captured much of the second crash was found after two years.
The additional information it helped reveal is disturbing.  A detailed article is here.

You can’t.  All you can do is hope the safety regulator is doing its job.
Someone with no prior flight training would have no idea what questions to even ask, much less a sound basis by which to judge any of the answers given.
For that reason alone, I would recommend against a “complete novice” partaking in such services.
Also, in my opinion, most of that (very expensive) experience would be wasted on someone who doesn’t have any clue what they’re doing or what is happening.

I am not a lawyer so I can not exactly tell you what is legal or not and don't have any experience with their service. But maybe I can help answer a part of your question.
The pilot that will fly with you is most likely an Instructor. So he has to teach people how to fly who never flew before. Flight students usually take control at their first flight once they are at a safe altitude. So I would not be to worried if they would actually let you take control of the airplane. If the airplane is high enough of the ground and they have declared a separate air space to perform their aerobatics maneuvers in it should be pretty save. The instructor is usually able to recover the plane from any maneuver given enough height. Air to air collisions would be avoided by having a separate area in which other planes are not allowed to fly in.
Given it is a true aerobatics plane it is unlikely you could damage the airplane structurally before you pass out.
Most importantly your instructor pilot should know what is safe and what is not and when to step in. Because he wants to get out of the plane alive as well.
So what you can do is to check the NOTAMs for the day to see if they designated a separate airspace. You can also ask to see the pilot's license and check if he is an instructor or ask him about his flight hours on the aircraft type. Lastly you could check out if the aircraft type is suited for aerobatics, which you won't need to do for the MIG-29 and the spitfire of course.


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How do the military pilots of Air Force One train for exceptional circumstances in an airliner not designed for high performance?
This article discusses how on one occasion, the pilot of Air Force One, the personal air transport of the President of the United States, was required to perform evasive manoeuvres to avoid presumed hostile fighter aircraft over Syria. In reality, the Syrian aircraft were friendlies and their military authorities had merely failed to properly communicate their intent. But nevertheless, it shows, and is logical, that the pilots of these converted civilian aircraft are required to learn and perform techniques that would never be used in the intended civilian operation of these aircraft.
How do pilots learn to fly an airliner, like the VC-137 in use at the time of this anecdote, or in the present day a 747, beyond its intended use and presumably with a high risk of failure? Does the United States Air Force retain identical aircraft not routinely used as Air Force One, for specific training use? Otherwise, how is it determined that these manoeuvres are feasible in such an aircraft and that the pilot is capable of performing them?

Air Force one is whatever air force plane the president happens to be in, not a particular plane.

Agreed, although I think @TomW is referring to the general modified 747 used to convey the president if possible.

Does the United States Air Force retain identical aircraft not
  routinely used as Air Force One, for specific training use?

The Airforce maintains two identical versions of the VC-25A that it uses to fulfill the role of Air Force One but its unclear (and likely not public) if any practice occurs in either airframe.

Otherwise, how is it determined that these manoeuvres are feasible in
  such an aircraft and that the pilot is capable of performing them?

Through testing, as is the case for the operational envelope of any aircraft. Due to the nature of Air Force One this data is not (as far as I know) public.
On any note the pilots, who are typically career Air Force pilots all have lots of experience and in some cases experience flying other heavy or high performance aircraft as is the case with one of the most recent pilots

Col. Tillman's distinguished career spans 30 years in the United
  States Air Force. Utilizing his Bachelors of Science degree in
  Chemical Engineering (Tulane Univ '79), he served as a Rocket
  Propulsion Engineer upon entering the service but was quickly selected
  to attend USAF pilot training. His flying experiences include time as
  a T-37 Instructor Pilot for undergraduate pilot training, C-130
  transport pilot, VIP airlift as a pilot in the Gulfstream III and IV

The air force trains for low altitude and evasive maneuvers in the C-130's not infrequently. I fly fairly close to a major air force base all the time, if you pick up flight following and get handed off to their tower you will often get a warning of such activity. You can generally see it out the window as well (its pretty neat). Presumably an force pilot coming from a C-130 background has some experience flying evasive maneuvers in heavy aircraft.


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For aircraft with variable-sweep wings, how unstable is it to have each wing in a different position?
Wikipedia presents this photograph of an F-14 Tomcat, with one variable-sweep wing swept forward and the other back:

I was surprised to see that this configuration was possible; much less feasible or safe to fly with. My idle assumption, not being an aviator of any kind, was that this would be extremely unstable and difficult to handle. I would have assumed, given the prior assumption that the aircraft would be unstable, that the wings would have interlocks requiring them to be swept symmetrically, as a safety feature.
Is this photograph of a maneuver that is manageable and safe enough to perform routinely, or would doing this with the wings be an exceptional event or done only for testing? What kind of impact would this have on the aircraft's handling, given a forewarned, experienced pilot?

This was an intentional test run by Grumman in response to US Navy concerns about asymmetric wing sweep.
From The story of F-14A, Aircraft No. 3, BuNo. 157982

One of these [flight tests] was in response to concerns raised by the US Navy regarding asymmetrical wing sweep. No. 3 is best remembered for photo shown at right. A series of flight tests were conducted from December 19, 1985 to February 28, 1986. Grumman's Chief Test Pilot, Chuck Sewell, conducted several trials with the right wing locked in the forward position of 20 degrees, and positioned the left wing at 35, 50, 60 and 68 degrees of sweep in flight. 60 degrees was determined as the maximum for landing. In the event of an operational in-flight malfunction, Sewell found the aircraft to be acceptable for carrier landings in this configuration.

Evidently it was manageable, and safe enough that carrier landings were permitted with asymmetric sweep.  There's no control available for the pilot of the Tomcat to intentionally asymmetrically-sweep the wings in flight, though.
How the handling qualities change I haven't seen reported anywhere.  I agree with Robert DiGiovanni that you'd expect additional drag on the side with the extended wing, but the Tomcat has additional options on how to move spoilers and other control surfaces and it isn't clear to me if it can detect and respond to asymmetric sweep or if it's just up to the pilot to do so.

I was the lead flight test engineer for Grumman on this project. The Navy had experienced a few mechanical failures that resulted in asymmetric wing sweep and this prompted a flight test program to determine the limits of controllability. We determined the aircraft could be flown slow enough where landing was feasible. Flaps were required to lower the approach airspeed to attempt any kind of shipboard landing, meaning both wings needed to be less than 50 degrees sweep. I think three aircraft recovered aboard ship with wings asymmetric to some extent and both forward of 50 degrees. We successfully landed at a field with wings at 20/60 and a fleet pilot eventually successfully landed at a field with a failure to the maximum possible split of 20/68.

Very cool -- welcome to Av.SE, and thanks for sharing that history here! Wonder what the ride felt like, with the wings that different.

Paul Conigliaro, I'm curious about one of the features of the F-14. Was it possible to lower the flaps with the wings swept back? Asking because that would make the aerodynamics very asymmetrical in this situation.

This appears to be a very fancy version of a side slip, which can be routinely performed with many aircraft.  Note the ailerons are set to roll left, with the outstretched right wing providing drag to yaw right.  It is doing the same job right rudder would do.
Slipping to the left would also be consistent with "raise the dead" thrust asymmetry technique if this was a twin with the right engine out.  Slipping to the left puts side force on the tail to counteract right yaw.
I would not do this particular maneuver too slow, particularly in that plane, where spin recovery would be more difficult.  But a well trained pilot would be in a safe enough envelope for a few photo shoot takes.

I'm not sure what you are calling an aileron, but it looks to me like the right wing spoiler is up to counter the extra lift being produced.  Also, your middle paragraph doesn't have anything to do with the question being asked.

@Michael Hall great comments Mike.  If you have any doubts, look at the left aileron.  Side slip is also used to control a twin engine out.  And no, spoiling lift and increasing drag on the extended wing (with all that torque) would be a no no. Notice the pilot is flying so well the rudders (in reserve) are still straight.

They might be spoiling the right wing a little bit.  Apparently that design did not make it to mass production.

The F-14 doesn't have ailerons. Instead, it uses wing spoilers at low speeds, and differential 'tailerons' at high speed

@Peter Kampf thanks, might have using those "televons" some too, and it would seem, in a slip to the left, the relative wind would lessen the sweep differential between the 2 wings, a "dihedral effect" from the swept left wing you have mentioned often in your writings.


text:
Does simulator time count as time in type?
Can full-motion simulator training time (FSI) be counted towards the required 100 hours for a Part 135 High Minimums Captain?

A substantially related question... although it does not have any answers as of yet.

Unfortunately, no.
135.225 - IFR: Takeoff, approach and landing minimums (e) requires:

The MDA or DA/DH and visibility landing minimums prescribed in part 97
  of this chapter or in the operator's operations specifications are
  increased by 100 feet and 1⁄2 mile respectively, but not to exceed the
  ceiling and visibility minimums for that airport when used as an
  alternate airport, for each pilot in command of a turbine-powered
  airplane who has not served at least 100 hours as pilot in command in
  that type of airplane.

So this means that our question becomes:  Can a pilot log PIC time when "flying" a simulator?
The answer to this question comes from 61.51 - Pilot logbooks, where you will notice that none of the bold parts (marked by me) have been met in the situation that you describe.  Generally, PIC time requires you to be PIC of an aircraft, and does not provide for PIC time when in a simulator unless you are providing instruction:

(e) Logging pilot-in-command flight time.
(1) A sport, recreational, private, commercial, or airline transport
  pilot may log pilot in command flight time for flights-
(i) When the pilot is the sole manipulator of the controls of an
  aircraft for which the pilot is rated, or has sport pilot privileges
  for that category and class of aircraft, if the aircraft class rating
  is appropriate;
(ii) When the pilot is the sole occupant in the aircraft;
(iii) When the pilot, except for a holder of a sport or recreational
  pilot certificate, acts as pilot in command of an aircraft for
  which more than one pilot is required under the type certification of
  the aircraft or the regulations under which the flight is conducted;
  or
(iv) When the pilot performs the duties of pilot in command while
  under the supervision of a qualified pilot in command provided—
...
(2) If rated to act as pilot in command of the aircraft, an airline
  transport pilot may log all flight time while acting as pilot in
  command of an operation requiring an airline transport pilot
  certificate.
(3) A certificated flight instructor may log pilot in command
  flight time for all flight time while serving as the authorized
  instructor in an operation if the instructor is rated to act as
  pilot in command of that aircraft.
(4) A student pilot may log pilot-in-command time only when the
  student pilot—
(i) Is the sole occupant of the aircraft or is performing the
  duties of pilot of command of an airship requiring more than one pilot
  flight crewmember;
(ii) Has a solo flight endorsement as required under §61.87 of this
  part; and
(iii) Is undergoing training for a pilot certificate or rating.

Don't forget the definition of aircraft in §1.1 -Aircraft means a device that is used or intended to be used for flight in the air.


text:
Do I need to register my drone?
Recently the FAA released some new regulations that require drone operators, including hobbyist, to register drones with the FAA.
As I am a novice in both aviation and legalese, can someone please break down the regulations and give me a guideline in layman's terms for whether or not I need to register a drone?

I hear the American Model Association (AMA) is in the process of suing the FAA because Congress told them not to mess with models...which your drone probably is.

@AaronD a drone user is suing the FAA but the AMA is just "discussing" with the FAA at this time.

@fooot Okay.  My point was the FAA has done something illegal in making this rule and so it may be struck down in court.  The specific parties involved are of lesser significance.

@AaronD it is not illegal at the moment, and may never be. The legality of it is for the courts to decide.

Is the FAA requiring registration of individual drones?  I thought the requirement was a registration of the operator (who is then allowed to operate the craft which meet the requirements), not actually each individual aircraft.  It's entirely possible that I am just misunderstanding though.

@loneboat From my understanding its individual drones, same as how every airplane needs to be registered.

The drone industry needs their own version of the NRA. :)

@DavidGrinberg: Hmmm, I interpreted it differently.  I went to the registration site and registered.  They issued me an ID number, and said the following:  "Once you receive a registration number, you can use it on all of your unmanned aircraft if they meet the online registration criteria. You must mark the registration number on all aircraft you own."  (from https://www.faa.gov/uas/registration/ )  Also note that they never once asked any specifics about a particular aircraft, only details about myself.  My perception was that I was registering MYSELF, not specific aircraft.

If:

Your unmanned aircraft weighs between 250g and 25kg (including fuel and cargo),
You are not flying under the Special Rule for Model Aircraft, also known as section 336
You intend to operate it outdoors, and
You are operating it in the United States,

you need to register your drone through the FAA's online registration system.  If your drone is heavier than 25kg, you need to go through the regular aircraft registration process.  If your drone is ligher than 250g, or you are only operating it indoors, you do not need to register it.
Additionally, if you are operating the drone for commercial purposes, you must have a remote pilot airman certificate.
(Note: if you are younger than 13 years old, someone who is 13 or older needs to register your drone for you.)

If I sell/transfer my drone, does it need to be re-registered?

According to the FAQ, yes.

A note: Even the FAA says "register your drone" or words to that effect. You will receive a single ID number that you can attach to all your aircraft. It really is to identify you as the owner, and helps ensure you will operate it in a responsible manner. You will want to ensure you remove the number before transferring ownership. Others may fly your craft. Note that there are certain restrictions, like not flying within 5 miles of an airport.

Although, I'm pretty sure my established flying club back in the 1990s was within a couple miles of Twin Oaks airport in Oregon. Nice runways, tables to work on planes, it was pretty nice.

This seems like a sensible precaution, anticipating a drone being recovered from inside a jet engine after a crash.

@Steve You can fly within 5 miles of an airport, but you must contact the tower prior to doing so.  They might tell you "no,", but it's typically so they can inform aircraft operating nearby of the activity.

You may want to add what the special rules are, because they apply to most folks who call their quad-copter a "drone". Under 400 ft, within sight, and not flying over large groups of people - no need to register since it's considered a model aircraft.

Well, everything in really basic terms can be found at the FAA Registration Site. Everything is there on that page.

Owners must register their UAS online if it meets the following guidelines:

Weighs more than 0.55 lbs. (250 g) and less than 55 lbs. (25 kg). Unmanned Aircraft weighing more than 55 lbs. cannot use this registration process and must register using the Aircraft Registry process.

THE RULES HAVE CHANGED.
Part 101 recreational [aka Model] operators, flying sUAS, are no longer required to register most drones under 55 lbs.  This is because the FAA is not enforcing recreational sUAS registrations.
The FAA is working on a new rule to reflect this.
Part 107 operators, will continue to need to register their sUAS.
This note:  Note: Model aircraft operators that comply with all of these operational requirements during flight do not have to register their UAS with the FAA.
Is on this FAA website page:  https://www.faa.gov/uas/getting_started/fly_for_fun/
NB: Probably this question should be deleted, and re-asked, given August 2016 FAA regulation changes, and the outcome of a subsequent court case, and yes, ultimately changes to the FAA regulations as a result of that case.

I would highly recommend following the AMA's guidance on this matter.  They've been doing this for many years now with safety as their primary mission before the FAA decided to stick their heavy hand into the matter after the increasing popularity of quad-rotor models led to several regrettable high-profile incidents by new flyers.
As of today (1/5/2016) the AMA is currently in negotiations with the FAA to achieve at the very least an exemption for AMA members, and their current advice on the matter is

Don't register yet as the deadline has not passed and you are not breaking the law until it does.
Submit comments to the FAA expressing your opinion of the new legislation.
Additional information will no doubt be posted on their blog as the situation evolves.

This hardly answers my question though, and when the deadline passes it wont answer it at all. I understand you may not agree with the regulations, but they exist and I'm asking under what conditions I need to follow them.

The real answer is that the current situation is in flux. You are NOT legally required to register any hobbyist models until February 19th, and whether you will be required to do so after then is at the moment is unclear one way or another.  The legality of the new regulations is questioned on several fronts, not just by the AMA.  It may not be an easy cut and dry answer as "you MUST register your drone if it is of such-and-such weight" but it most accurately reflects the reality of the current situation.

As long as your drone is not on the list, you are exempted, if you are really worried about your drone you might want to see the list here from this website http://smalldronesreview.com/2015/12/28/list-of-exempted-drones-for-registration-not-required/
But lately there are moves about abolishing the regulations on registration of drone, but if you have one especially large drones, just to be safe register it follow the law or be sorry